3,138 research outputs found
Numerical studies towards practical large-eddy simulation
Large-eddy simulation developments and validations are presented for an
improved simulation of turbulent internal flows. Numerical methods are proposed
according to two competing criteria: numerical qualities (precision and
spectral characteristics), and adaptability to complex configurations. First,
methods are tested on academic test-cases, in order to abridge with fundamental
studies. Consistent results are obtained using adaptable finite volume method,
with higher order advection fluxes, implicit grid filtering and "low-cost"
shear-improved Smagorinsky model. This analysis particularly focuses on mean
flow, fluctuations, two-point correlations and spectra. Moreover, it is shown
that exponential averaging is a promising tool for LES implementation in
complex geometry with deterministic unsteadiness. Finally, adaptability of the
method is demonstrated by application to a configuration representative of
blade-tip clearance flow in a turbomachine
CP Violation in Hyperon Nonleptonic Decays within the Standard Model
We calculate the CP-violating asymmetries A(Lambda_-^0) and A(Xi_-^-) in
nonleptonic hyperon decay within the Standard Model using the framework of
heavy-baryon chiral perturbation theory (chiPT). We identify those terms that
correspond to previous calculations and discover several errors in the existing
literature. We present a new result for the lowest-order (in chiPT)
contribution of the penguin operator to these asymmetries, as well as an
estimate for the uncertainty of our result that is based on the calculation of
the leading nonanalytic corrections.Comment: 21 pages, 2 figures; discussion clarified, results & conclusions
unchanged, to appear in Phys. Rev.
Genomic Content of Bordetella pertussis Clinical Isolates Circulating in Areas of Intensive Children Vaccination
BACKGROUND: The objective of the study was to analyse the evolution of Bordetella pertussis population and the influence of herd immunity in different areas of the world where newborns and infants are highly vaccinated. METHODOLOGY: The analysis was performed using DNA microarray on 15 isolates, PCR on 111 isolates as well as GS-FLX sequencing technology on 3 isolates and the B. pertussis reference strain, Tohama I. PRINCIPAL FINDINGS: Our analyses demonstrate that the current circulating isolates are continuing to lose genetic material as compared to isolates circulating during the pre-vaccine era whatever the area of the world considered. The lost genetic material does not seem to be important for virulence. Our study confirms that the use of whole cell vaccines has led to the control of isolates that were similar to vaccine strains. GS-FLX sequencing technology shows that current isolates did not acquire any additional material when compared with vaccine strains or with isolates of the pre-vaccine era and that the sequenced strain Tohama I is not representative of the isolates. Furthermore, this technology allowed us to observe that the number of Insertion Sequence elements contained in the genome of the isolates is temporally increasing or varying between isolates. CONCLUSIONS: B. pertussis adaptation to humans is still in progress by losing genetic material via Insertion Sequence elements. Furthermore, recent isolates did not acquire any additional material when compared with vaccine strains or with isolates of the pre-vaccine era. Herd immunity, following intensive vaccination of infants and children with whole cell vaccines, has controlled isolates similar to the vaccine strains without modifying significantly the virulence of the isolates. With the replacement of whole cell vaccines by subunit vaccines, containing only few bacterial antigens targeting the virulence of the bacterium, one could hypothesize the circulation of isolates expressing less or modified vaccine antigens
New Physics and CP Violation in Hyperon Nonleptonic Decays
The sum of the CP-violating asymmetries A(Lambda_-^0) and A(Xi_-^-) in
hyperon nonleptonic decays is presently being measured by the E871 experiment.
We evaluate contributions to the asymmetries induced by chromomagnetic-penguin
operators, whose coefficients can be enhanced in certain models of new physics.
Incorporating recent information on the strong phases in Xi->Lambda pi decay,
we show that new-physics contributions to the two asymmetries can be
comparable. We explore how the upcoming results of E871 may constrain the
coefficients of the operators. We find that its preliminary measurement is
already better than the epsilon parameter of K-Kbar mixing in bounding the
parity-conserving contributions.Comment: 12 pages, 2 figure
MXene molecular sieving membranes for highly efficient gas separation
Molecular sieving membranes with sufficient and uniform nanochannels that break the permeability-selectivity trade-off are desirable for energy-efficient gas separation, and the arising two-dimensional (2D) materials provide new routes for membrane development. However, for 2D lamellar membranes, disordered interlayer nanochannels for mass transport are usually formed between randomly stacked neighboring nanosheets, which is obstructive for highly efficient separation. Therefore, manufacturing lamellar membranes with highly ordered nanochannel structures for fast and precise molecular sieving is still challenging. Here, we report on lamellar stacked MXene membranes with aligned and regular subnanometer channels, taking advantage of the abundant surface-terminating groups on the MXene nanosheets, which exhibit excellent gas separation performance with H₂ permeability >2200 Barrer and H₂/CO₂ selectivity >160, superior to the state-of-the-art membranes. The results of molecular dynamics simulations quantitatively support the experiments, confirming the subnanometer interlayer spacing between the neighboring MXene nanosheets as molecular sieving channels for gas separation.Li Ding, Yanying Wei, Libo Li, Tao Zhang, Haihui Wang, Jian Xue, Liang-Xin Ding, Suqing Wang, Jürgen Caro, Yury Gogots
Spectrally-resolved UV photodesorption of CH4 in pure and layered ices
Context. Methane is among the main components of the ice mantles of
insterstellar dust grains, where it is at the start of a rich solid-phase
chemical network. Quantification of the photon-induced desorption yield of
these frozen molecules and understanding of the underlying processes is
necessary to accurately model the observations and the chemical evolution of
various regions of the interstellar medium. Aims. This study aims at
experimentally determining absolute photodesorption yields for the CH4 molecule
as a function of photon energy. The influence of the ice composition is also
investigated. By studying the methane desorption from layered CH4:CO ice,
indirect desorption processes triggered by the excitation of the CO molecules
is monitored and quantified. Methods. Tunable monochromatic VUV light from the
DESIRS beamline of the SOLEIL synchrotron is used in the 7 - 13.6 eV (177 - 91
nm) range to irradiate pure CH4 or layers of CH4 deposited on top of CO ice
samples. The release of species in the gas phase is monitored by quadrupole
mass spectrometry and absolute photodesorption yields of intact CH4 are
deduced. Results. CH4 photodesorbs for photon energies higher than ~9.1 eV
(~136 nm). The photodesorption spectrum follows the absorption spectrum of CH4,
which confirms a desorption mechanism mediated by electronic transitions in the
ice. When it is deposited on top of CO, CH4 desorbs between 8 and 9 eV with a
pattern characteristic of CO absorption, indicating desorption induced by
energy transfer from CO molecules. Conclusions. The photodesorption of CH4 from
the pure ice in various interstellar environments is around 2.0 x 10^-3
molecules per incident photon. Results on CO-induced indirect desorption of CH4
provide useful insights for the generalization of this process to other
molecules co-existing with CO in ice mantles
Haemodynamics and flow modification stents for peripheral arterial disease:a review
Endovascular stents are widely used for the treatment of peripheral arterial disease (PAD). However, the development of in-stent restenosis and downstream PAD progression remain a challenge. Stent revascularisation of PAD causes arterial trauma and introduces abnormal haemodynamics, which initiate complicated biological processes detrimental to the arterial wall. The interaction between stent struts and arterial cells in contact, and the blood flow field created in a stented region, are highly affected by stent design. Spiral flow is known as a normal physiologic characteristic of arterial circulation and is believed to prevent the development of flow disturbances. This secondary flow motion is lost in atheromatous disease and its re-introduction after endovascular treatment of PAD has been suggested as a method to induce stabilised and coherent haemodynamics. Stent designs able to generate spiral flow may support endothelial function and therefore increase patency rates. This review is focused on secondary flow phenomena in arteries and the development of flow modification stent technologies for the treatment of PAD
Design and construction of the MicroBooNE Cosmic Ray Tagger system
The MicroBooNE detector utilizes a liquid argon time projection chamber
(LArTPC) with an 85 t active mass to study neutrino interactions along the
Booster Neutrino Beam (BNB) at Fermilab. With a deployment location near ground
level, the detector records many cosmic muon tracks in each beam-related
detector trigger that can be misidentified as signals of interest. To reduce
these cosmogenic backgrounds, we have designed and constructed a TPC-external
Cosmic Ray Tagger (CRT). This sub-system was developed by the Laboratory for
High Energy Physics (LHEP), Albert Einstein center for fundamental physics,
University of Bern. The system utilizes plastic scintillation modules to
provide precise time and position information for TPC-traversing particles.
Successful matching of TPC tracks and CRT data will allow us to reduce
cosmogenic background and better characterize the light collection system and
LArTPC data using cosmic muons. In this paper we describe the design and
installation of the MicroBooNE CRT system and provide an overview of a series
of tests done to verify the proper operation of the system and its components
during installation, commissioning, and physics data-taking
Ionization Electron Signal Processing in Single Phase LArTPCs II. Data/Simulation Comparison and Performance in MicroBooNE
The single-phase liquid argon time projection chamber (LArTPC) provides a
large amount of detailed information in the form of fine-grained drifted
ionization charge from particle traces. To fully utilize this information, the
deposited charge must be accurately extracted from the raw digitized waveforms
via a robust signal processing chain. Enabled by the ultra-low noise levels
associated with cryogenic electronics in the MicroBooNE detector, the precise
extraction of ionization charge from the induction wire planes in a
single-phase LArTPC is qualitatively demonstrated on MicroBooNE data with event
display images, and quantitatively demonstrated via waveform-level and
track-level metrics. Improved performance of induction plane calorimetry is
demonstrated through the agreement of extracted ionization charge measurements
across different wire planes for various event topologies. In addition to the
comprehensive waveform-level comparison of data and simulation, a calibration
of the cryogenic electronics response is presented and solutions to various
MicroBooNE-specific TPC issues are discussed. This work presents an important
improvement in LArTPC signal processing, the foundation of reconstruction and
therefore physics analyses in MicroBooNE.Comment: 54 pages, 36 figures; the first part of this work can be found at
arXiv:1802.0870
A Deep Neural Network for Pixel-Level Electromagnetic Particle Identification in the MicroBooNE Liquid Argon Time Projection Chamber
We have developed a convolutional neural network (CNN) that can make a
pixel-level prediction of objects in image data recorded by a liquid argon time
projection chamber (LArTPC) for the first time. We describe the network design,
training techniques, and software tools developed to train this network. The
goal of this work is to develop a complete deep neural network based data
reconstruction chain for the MicroBooNE detector. We show the first
demonstration of a network's validity on real LArTPC data using MicroBooNE
collection plane images. The demonstration is performed for stopping muon and a
charged current neutral pion data samples
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